In general, when the rubber composition is exposed to a higher temperature during the vulcanization of the rubber composition or in the actual use as a vulcanized rubber for tires and the like, crosslinked portions among rubber segments mainly formed by the vulcanization are broken to bring about the deterioration of rubber. As means for improving a resistance to rubber deterioration in the exposure to the higher temperature (heat resistance) is effective a technique of making a structure of the crosslinked portion among rubber segments to a bond having a large dissociation energy. Concretely, there is mentioned a technique of reducing an amount of sulfur compounded and increasing an amount of a vulcanization accelerator compounded (so-called EV system) because the heat resistance of the crosslinked portion is increased in the order from polysulfide bond (CSx-C) through disulfide bond (C—S—S—C) to monosulfide bond (C—S—C). In this technique, however, the heat resistance of rubber is improved, but the bonding length in the crosslinked portion among rubber segments becomes short and there is a problem of deteriorating an elongation at break (Eb) or a tenacity at break (Tb).
In connection with the trend of worldwide regulation on the discharge of carbon dioxide associated with the escalation in interest on recent environmental issues, a demand on low fuel consumption of vehicles is strongly increasing. In order to cope with such a demand, it is required to reduce the rolling resistance as a tire performance. As a technique for reducing the rolling resistance of the tire, it is effective to use a rubber composition having a lower loss tangent (tan δ) and a low heat buildup as a rubber composition applied to a tread portion of the tire.
On the other hand, a rubber composition having a high storage elastic modulus (G′) is suitable as a rubber composition applied to a tread portion, a sidewall portion, a bead filler and the like in the tire, so that it is required to develop a rubber composition having a low loss tangent (tan δ) and a high storage elastic modulus (G′). As means for increasing the storage elastic modulus (G′) of the rubber composition, there are known a technique of increasing an amount of carbon black compounded in the rubber composition, a technique of compounding bismaleimide (BMI) with a particular structure such as N,N′-(4,4′-diphenylmethane)-bismaleimide or the like as disclosed in JP-A-2002-121326, and a technique of compounding a compound having a reactive group to a rubber component and an adsorption group to a filler such as polyethylene glycol dimaleate (PEGM) or the like as disclosed in JP-A-2003-176378.
However, when the amount of carbon black compounded in the rubber composition is increased, the storage elastic modulus (G′) of the rubber composition can be improved, but the loss tangent (tan δ) of the rubber composition is simultaneously increased to deteriorate the low heat buildup of the rubber composition, and further the Mooney viscosity of the rubber composition is increased to deteriorate the processability.
When the compound having a reactive group to bismaleimide (BMI) or a rubber component and an adsorption group to a filler is compounded into the rubber composition, the storage elastic modulus (G′) of the rubber composition can be improved, the loss tangent (tan δ) of the rubber composition is substantially equal to that of the rubber composition having no compound and hence the low heat buildup of the rubber composition can not be improved sufficiently.